CancelledError not showing up as a backtrace

Canceling a task or raising `CancelledError` does not produce a backtrace. This is true when using the Python `Task` implementation. When tested on MicroPython, which uses the native `_uasyncio.Task` implementation, this works properly.

(I also tried getting the native `_uasyncio` up and running in CircuitPython, but it's also having issues, because we require that it provide an `Task.__await__()`, which it does not provide.)

Start with the Python Task implementation and the CancelledError handling, then compare its cancellation behavior with the native _uasyncio.Task implementation described in the issue. Reproduce a cancelled task or raised CancelledError and verify that a backtrace is produced; also account for the noted Task.__await__() difference in CircuitPython.

Written by the indexing model from the issue text.